Methyl conjugation is an important reaction in the biotransformation of many drugs, nutrients, other xenobiotics, neurotransmitters and hormones. The applicant's laboratory is engaged in a systematic exploration of the pharmacogenetics of "small molecule" methylation. To achieve that goal, we have applied the techniques of molecular biology and genomics to clone and characterize genes encoding methyltransferase (MT) enzymes in humans;have identified single nucleotide polymorphisms (SNPs) and other DNA sequence variation of importance for individual variation in methylation;and have determined underlying mechanisms responsible for those functional effects. This approach has resulted in the identification and characterization of a series of common, biologically and medically significant genetic polymorphisms for MT enzymes such as those for the thiopurine drug-metabolizing enzyme thiopurine S- methyltransferase (TPMT) and the catecholamine and catechol drug-metabolizing enzyme catechol O- methyltransferase (COMT). Common "pharmacogenetic" variation in the genes encoding these enzymes contributes significantly to individual differences in response to drug therapy and/or the pathophysiology of disease. During the next funding cycle, we plan to study a newly described and potentially important arsenic methylating enzyme, AS3MT. However, we also propose to expand our studies of the role of inheritance in individual variation in methyl conjugation to focus our major effort on the pharmacogenetics of the "AdoMet- Methionine Cycle", the cyclic pathway that regenerates S-adenosyl-L-methionine (AdoMet), the methyl donor cosubstrate for most small molecule MTs. Specifically, we plan to apply a genotype-to-phenotype pharmacogenetic research strategy to this pathway. The steps in that process will begin with the resequencing of genes encoding enzymes in this pathway, followed by functional genomic and mechanistic studies as well as genotype-phenotype correlation analyses. The results of these experiments will help us to link variation in genotype to variation in phenotype for this important metabolic pathway and to increase our understanding of molecular mechanisms responsible for that genotype-phenotype correlation. They will also help us understand role of the AdoMet-Methionine Cycle in individual variation in response to drug therapy, as well as its contribution to disease risk.